Automatic derivation of Codec for Scala 3 union types

It would be great if Circe could automatically derive Encoder/Decoder for Scala union types, for example,
```scala
given [A: Decoder, B: Decoder]: Decoder[A | B] = 
  Decoder[A] or Decoder[B].widen

given [A: Encoder, B: Encoder]: Encoder[A | B] =
  Encoder.instance:
    case a: A => Encoder[A].apply(a)
    case b: B => Encoder[B].apply(b)

```